This invention relates to an energy storage system, and more particularly to a battery load levelling system.
A common problem for electric utilities is that demand for electrical energy is at a peak during certain hours of the day, typically between 10:00 a.m. and 3:00 p.m. Demand for electrical energy then tails off significantly during the late afternoon and evening hours, and is at its lowest during the early morning hours. A problem with this fluctuating demand for electrical energy is that, with most types of electric power plants, it is undesirable to operate such plants according to the fluctuating demand for electricity. That is, most generating plants operate optimally at a constant level of output. As a result, many electric utilities offer discounts for electricity consumed during periods of off-peak demand as an incentive for users to adjust their patterns of consumption, and thereby the load demand curve.
One way to alleviate the problems associated with fluctuating electrical demand is to store electricity produced by the generating facility during the off-peak hours, and selectively discharge such stored electricity during peak hours. This may be done either by the utility as a means for meeting increased demand during peak hours, or by a customer for taking advantage of the lower rates associated with consumption during off-peak hours. Either way the peaks and valleys in the energy demand curve are somewhat smoothed by the existence of the storage facility.
One such technology for storing electrical power produced during off-peak hours is a battery storage system. In this type of system, a large number of battery cells are charged with power produced during off-peak hours of demand, which is released to accommodate demand during peak hours. The cycle then repeats as the batteries are charged during off-peak hours and again discharged during peak hours.
A facility which utilizes the described battery storage technology is the Battery Energy Storage Test (BEST) facility in Somerset County, New Jersey. In accordance with the technology developed to date, the BEST facility utilizes a single power converter which converts the AC power supplied by the utility into DC power, which is transferred to a large number of lead-acid battery cells for storage of the DC power. To output power from the facility, DC power is transferred from the lead-acid batteries back to the single converter, and into the utility line as AC power.
A problem with the described form of battery storage system lies in the fact that the AC power input from the utility is converted at a single central converter to DC power, which is then transferred via buses, switching and other transfer components to the DC battery cells for storage. Accordingly, the wiring and switch gear between the converter and the battery cells must be constructed to handle DC power. A further problem with this type of system is that the terminals of the banks of battery cells used to store the electrical power are exposed, creating a danger to personnel in the facility as well as frequent maintenance. Yet another drawback to this type of system is that, for each application, the quantity of battery cells and the wiring, switching and other components must be designed according to the specific required capacity of a given installation. If different installations specify different capacities, then the components must be redesigned anew.
The present invention is designed to eliminate or overcome problems inherent in the above-described battery storage system. In accordance with the invention, a modular electrical load levelling system comprises a plurality of modules including means for storing DC electrical power. In one embodiment, such means for receiving and storing DC electrical power may comprise a plurality of conventional lead-acid battery cells. Power conversion means is associated with each module for converting input AC power to DC power, which is then stored in the battery cells. The power conversion means also converts output DC power from the battery cells to AC power. Power transfer means is associated with each module for inputting and outputting AC power to and from the module. Control means is provided for controlling the supply of electrical power to the system and the output of electrical power from the system. The control means, which may be a personal computer or other satisfactory electrical or electronic circuitry, is interconnected with each module for controlling the supply of power thereto and the output of power therefrom. In response to the control means, the DC power stored in the battery cells contained within each module is converted to AC power for output. The control means also acts to control the supply of power for charging the battery cells, which is preferably during periods of off-peak power demand. The control means may incorporate any satisfactory mechanism for triggering supply or discharge of power to or from the system. For example, a conventional timer clock, set according to peak and off-peak periods of demand, may be used in connection with the control means for controlling supply or output of power to or from the system.
Also in accordance with the invention, each module comprises an enclosure adapted to house one or more of the battery cells. The power conversion means associated with each module is also housed within the enclosure, to thereby provide a single modular unit capable of having both an AC power input and output. For this reason, each separate module may be termed an "AC battery". AC power is supplied directly to each module, where it is converted by the power conversion means to DC power for storage in the battery cells. To output electrical power, the DC power is processed through the power conversion means to AC power, which is then output from the module. The modules can efficiently be factory produced en masse, and the connections between the battery cells and the power converter can likewise be factory made to eliminate or greatly reduce the need for maintenance thereof.
The battery load levelling system of the invention preferably incorporates a plurality of modules as above described. AC power is output from each individual module, which thereby eliminates the necessity for DC connections, wiring and hardware usually associated with batteries. Further, the terminals of each DC battery cell are concealed within the enclosure which makes for a clean and safe environment for the system.
An important advantage of the AC battery of the invention is that the power terminals may be shorted together without damaging the module or creating a personnel hazard. This is far different than a DC battery, which can typically supply very high electrical currents and/or rupture when shorted, thereby creating potential for serious personal injury. The AC battery terminals can be shorted due to the utilization of solid state switches, which are normally open, between the stored DC energy and the AC terminals.
The wiring and connections for the series of modules can be conventional AC wiring, which is inherently safer than DC. Further, the switching and other necessary electrical components are also AC type components, which make field installation much safer than in DC applications.
The modular construction of the battery load levelling system of the invention provides reduced engineering and other one-time costs associated with installation of such a system. That is, for different capacities desired by a user, the user needs only purchase enough modules to accommodate this capacity. If another user requires a different capacity, a satisfactory number of modules is provided. The only practical variation between installations of different capacities is the number of modules required.
The invention also contemplates an efficient and convenient arrangement for storing and stacking the plurality of modules utilized in the system, which also addresses the problem of conduit and cable placement. In accordance with this aspect of the invention, rack means is utilized to support the plurality of modules. A plurality of conduits are provided to supply AC power to and discharge AC power from the modules. The rack means is provided with conduit support means for supporting the conduits. In one embodiment, the rack means includes a series of vertical and horizontal support members for supporting the modules. The conduit support means is incorporated into the support members, and in one embodiment the conduits are physically placed inside the support members. With the construction, separate trays and raceways for the conduit and cables are unnecessary, and the routing of such components inside the rack structure provides a less cluttered, less costly system.